Date of Award

8-2015

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Ramakrishna Guda

Second Advisor

Dr. Sherine Obare

Third Advisor

Dr. Ekkehard Sinn

Fourth Advisor

Dr. Massood Atashbar

Keywords

Fluorescence Upconversion, Third harmonic, Laser Spectroscopy ultrafast fluorescence, semiconductor nanooarticle, ZnO nanooarticles

Abstract

Charge-carrier relaxation dynamics in semiconductor nanoparticles has been in the forefront of research for last three decades both for their fundamental nature as well as for applications in light-harvesting, luminescent labels, light-emitting diodes and nonlinear optics. Significant research has focused on II-VI semiconductors with emphasis on CdS and CdSe that are toxic. On the other hand, ZnO and TiO2 semiconductor nanoparticles are less toxic and have enormous applications in photo-catalysis, water splitting, solar cells and optical sensors. However, the investigations of charge-carrier dynamics in these semiconductor nanomaterials compared to II-VI semiconductor quantum dots are quite limited. The knowledge of charge-carrier relaxation is essential to realize their optical applications. In this dissertation, efforts were made to probe ultrafast charge-carrier dynamics in bare ZnO and surface-modified TiO2 nanoparticles with combined femtosecond fluorescence upconversion and transient absorption measurements.

Quantum-sized ZnO semiconductor nanoparticles show two luminescence bands corresponding to band-edge and trap-state luminescence where the dynamics of band-edge luminescence was not well understood. In this dissertation, studies were carried out to probe the ultrafast band-edge luminescence dynamics in bare ZnO nanoparticles as a function of size. The measurements have shown interesting role of rate of trapping in the band-edge luminescence dynamics and conclusively shown that trapping dynamics dominates the charge-carrier relaxation. Along similar lines, ultrafast interfacial charge-transfer emission was monitored in small molecule-modified TiO2 nanoparticles and have shown that small molecules create inter band-gap charge-transfer excitons that delocalize relatively fast into the conduction band of TiO2 leading to efficient charge separation. In this study, results revealed that increasing the strength of donor leads to better charge separation. Ultrafast measurements on chromophore-surface modified TiO2 nanoparticles have shown the role of localized charge-transfer excitons in charge separation and their use in dye solar cells. The results have shed light on heterogeneous interfacial charge-carrier relaxation dynamics in dye functionalized semiconductor nanoparticles. Finally, interesting trap-state luminescence of ZnO nanoparticles was used to develop turn-on luminescent dissolved oxygen (DO) sensors. Further, the sensitivity of ZnO nanoparticles towards DO was improved by preparing ZnO/graphene oxide nanocomposites that have shown additional ratiometric sensing capabilities for DO

Access Setting

Dissertation-Open Access

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